Malaria is a growing public health problem in many developing countries. This disease, which is caused by protozoan parasites of the genus Plasmodium and transmitted by anopheline mosquitoes, continues to resist all conventional forms of control. Current strategies for vector control and malaria vaccine development are focusing on specific stages in the Plasmodium life cycle; several stages which occur in the vector are thought to be especially vulnerable to interruption. In nature, factors regulating vector infection rates, sporozoite loads (infective-stage sporozoites in the salivary glands) and sporozoite transmission are largely unknown. Comprehensive approaches are needed for evaluating Anopheles vector potential. The proposed studies will investigate the biology of Plasmodium development in Anopheles vectors to determine factors affecting sporozoite transmission potential. Anopheles gambiae, a highly competent African malaria vector, will be infected with in vitro cultured gametocytes of P. falciparum, the most pathogenic human malaria. Variability of infection and sporozoite transmission will be evaluated by 1) quantifying sequential sporogonic stages in the vector (ingested gametocytes, ookinetes, oocysts and sporozoites) 2) quantifying sporozoite dissemination (e.g. in the hemocoel, salivary glands, salivary duct, etc.), and 3) determining the number of sporozoites transmitted during feeding, using in vitro methods standardized against an in vivo mouse model. This will establish a foundation for investigating the effects of life history and behavior patterns of An. gambiae (e.g. adult feeding history, adult body size, feeding behavior, gonotrophic cycles, exposure to multiple infectious hosts, etc.) on susceptibility to P. falciparum infections, sporogonic development and transmission potential. Plasmodium falciparum infections will be studied further in An. arabiensis, An. dirus, An. freeborni and An. stephensi, all competent vectors, to identify and compare critical points affecting vector potential. By focusing on regulatory mechanisms for vector potential, this project will yield new information for studying natural malaria transmission, for evaluating malaria control programs, and for developing and field testing malaria vaccines.